The present invention relates generally to multichannel record disc recording systems, and more particularly to a system which, automatically controls the level of an angle-modulated wave signal in accordance with the level of a direct wave signal. The direct wave signal is superimposed on an angle-modulated wave signal and recorded, as thus superimposed, on a record disc.
A 4-channel record disc recording system was previously described in U.S. Pat. Nos. 3,686,471 and 3,883,699, which has already been reduced to practice signals from two channels are matrixed thereby to obtain, respectively, sum and difference signals. Then, a carrier wave is angle modulated with the difference signal. The resulting angle-modulated difference signal is multiplexed with a direct wave sum signal, and the signals thus multiplexed are recorded on one sidewall of a groove on the record disc. Another two channels are recorded on the other sidewall, in a similar manner.
If the levels of the respective channel signals are high, interference will readily occur between the high-frequency band component and the angle-modulated wave difference signal of the direct wave sum signal, at the time of recording of the 4-channel record disc. A similar interference may also occur at the time of reproducing by means of a pickup cartridge. In such a case, the interference will give rise to various problems such as disturbance noise, impairment of the signal-to-noise (S/N) ratio, and deterioration of the distortion factor.
Accordingly, one conceivable measure for preventing these adverse effects is to keep the angle-modulated wave difference signal at a high level. However, when the level of the angle-modulated wave difference signal is so maintained at a high value, the cutter head generates heat while cutting and recording on the disc. The heat requires special gas cooling means of an elaborate scale and high cost.
Accordingly, we have previously described a system intended to solve these problems. That system is disclosed in U.S. Pat. No. 3,806,667, entitled FOUR CHANNEL PHONOGRAPH MULTIPLEX RECORDING SYSTEM WITH SIGNAL LEVEL CONTROL. This system has a device for reproducing a magnetic tape on which program signals are recorded. An auxiliary head is provided ahead or upstream of the regular reproducing head, as viewed in the tape traveling direction. A signal reproduced by this preceding head is passed through a number of circuits, such as an equalizer circuit, a gate circuit, a delay circuit, and an integration circuit, to obtain a control signal which varies in accordance with the level of the reproduced signal. By this control signal, the level of the angle-modulated wave difference signal is automatically controlled.
This system requires a separate transmission line for the regularly reproduced 4-channel signals and for the control signal obtained from the preceding auxiliary head. For this reason, it is necessary to readjust the level of the control signal where the signal level (cutting level) is set in the 4-channel signal transmission system. Furthermore, this previous system requires the preceding auxiliary head, and the magnetic tape reproducing device becomes complicated. In addition, there are problems caused by the complicated nature of the circuit system required to obtain the above mentioned control signal. It is necessary to use a disc cutting apparatus which is large and elaborate, and is troublesome to adjust.